1. Field of the Invention
This invention relates to heat-sensitive recording elements for making color images and to a method of making color images using said elements.
2. Description of the Prior Art
A variety of thermal imaging systems for producing color images have been proposed. One system commonly employed for heat-sensitive recording materials comprises a two-component system utilizing a heat induced coloration reaction between an electron donating compound (color former) and an electron accepting compound (color developer). For producing dye images, the electron donating compound usually is a colorless electron donating dye comprising a triarylmethane, diphenylmethane, xanthene, thiazine or spiro compound, for example, Crystal Violet Lactone, N-halophenyl leuco Auramine, rhodamine B anilinolactam, 3-piperidino-6-methyl-7-anilinofluoran, benzoyl leuco Methylene blue, 3-methyl-spirodinaphthofuran, etc. The electron accepting compound is an acidic material usually, a phenol derivative or an aromatic carboxylic acid derivative, for example, p-tert-butylphenol, 2,2-bis (p-hydroxyphenyl)propane, 1,1-bis(p-hydroxyphenyl) pentane, p-hydroxybenzoic acid, 3,5-di-tert-butylsalicylic acid, etc. Such thermal imaging materials and various combinations thereof are now well known, and various methods of preparing heat-sensitive recording elements employing these materials also are well known and have been described, for example, in U.S. Pat. Nos. 3,539,375, 4,401,717 and 4,415,633.
Heat-sensitive recording materials employing two-component systems such as the foregoing ordinarily include a binder or some other means for physically separating the two components to prevent premature mixing and coloration and usually are prepared by dispersing the two components as fine particles in a binder and then coating this mixture on a support. Images are formed by applying heat to melt one or both of the components so that they will come into contact with each other to form color. Depending upon the colorless electron-donating dye, the coloration reaction initiated by contacting the melted component(s) may comprise dissociation or ring-opening in those compounds containing a cyclic ring-closing moiety as part of their structure. For example, color formers such as triarylmethane compounds possessing a lactone or lactam moiety ring-closed on the methane carbon atom become ring-opened and colored when contacted with the acidic electron accepting compound by an environmental reaction usually an ionization or hydrogen-bonding reaction.